a) Field of the Invention
The present invention relates to a nearly telecentric retrofocus type objective lens system which is to be used in fiber scopes and video scopes.
b) Description of the Prior Art
Known as conventional examples of retrofocus type objective lens systems for endoscopes are the lens systems which are described below:
The objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 55-55,308, for example, consists, in order from the object side, of a negative lens unit, an aperture stop, a positive lens unit and a cemented lens unit, or is composed as a whole of four lens elements. This conventional example is configured so as to correct chromatic aberration and spherical aberration by using the cemented lens unit. However, this objective lens system forms images which are not so bright and allows spherical aberration to remain at least on the order of 5% of a focal length of the lens system as a whole when it is specified for F/3.5. It is difficult to configure this type of objective lens system so as to form bright images by reducing the F number thereof.
Further, the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 57-64,207, is configured so as to correct aberrations when it has a wide field angle and has spherical aberration which is corrected rather favorably, but requires a high manufacturing cost due to a fact that the lens system comprises five or six lens elements.
Furthermore, Japanese Patent Kokai Publication No. Hei 2-69,710 discloses, as certain embodiments thereof, objective lens systems each of which consists of a negative lens unit, an aperture stop and a positive lens unit. Each of the lens systems preferred as the above-mentioned embodiments consists of three lens elements, but has an F number of 3 or larger, forms dark images and allows negative spherical aberration to remain therein, whereby the lens system can hardly form bright images or have an aperture larger than F3.
As is understood from the foregoing description, each of the conventional telecentric retrofocus type objective lens systems has a common defect that the lens systems form dark images with large F numbers or that the lens systems comprise large numbers of lens elements.
Moreover, there is known no retrofocus type objective lens system for endoscopes which has a numerical aperture enlarged by using an aspherical surface though objective lens systems using aspherical surfaces are known to those skilled in the art.